A. Chemical Analysis of Body Fluids
The determination of a patient's physiological status is frequently assisted by a chemical analysis of a body fluid of that patient. Such an analysis normally determines the existence and/or concentration of a chemical species in the body fluid of the patient as an indication of that patient's condition. Increasingly, such analyses are being used to determine the presence of a drug of addiction or a metabolite of such a drug in the body fluid of an individual.
Many analytes of interest can be detected in urine, which is readily available from a subject and can be collected non-invasively. For these reasons, the primary method for detecting drugs of abuse today is urine analysis.
Blood, however, is also frequently analyzed for the presence of drugs of addiction as well as a wide variety of other analytes. Blood collection is, however, inherently invasive, and carries the risk of infection associated with any invasive procedure. Blood testing must also be conducted at a physician's office or at another facility equipped to analyze blood, which reduces the convenience of blood tests and increases their cost. In addition, testing a blood sample can only reveal information about chemicals or metabolites that are present in the blood of the subject at the time the sample is taken, and cannot detect the presence of such analytes over a period of time.
Perspiration can also be collected in order to analyze a chemical species present in the body. The non-invasive manner in which it can be collected renders perspiration suitable for use outside of a physician's office. In addition, a variety of molecules which are expressed in perspiration can be analyzed.
B. Diagnostic Kits for Collecting Perspiration
A variety of diagnostic kits for monitoring an analyte in sweat have been developed. For example, U.S. Pat. No. 3,552,929 to Fields, et al. discloses a band-aid-type test patch suited for determining the chloride ion concentration in perspiration as a method of diagnosing cystic fibrosis. The apparatus disclosed in Fields comprises an absorptive sweat collecting pad with an impermeable overlying layer for the purpose of preventing evaporation. When the absorptive pad is saturated, the patch is removed from the skin and exposed to a series of strips impregnated with incremental quantities of silver chromate or silver nitrate, the color of which undergoes a well known change upon conversion to the chloride salt.
U.S. Pat. No. 4,706,676 to Peck discloses a dermal collection device which comprises a binder to prevent reverse migration of an analyte, a liquid transfer medium which permits transfer of an analyte from the dermal surface to the binder, and an occlusive cover across the top of the liquid transfer medium and binder. Peck also discloses the application of such a dermal collection patch to detect various environmental chemicals to which humans are exposed. After the dermal collection device has been worn on a patient's skin for a period of time, the patch is removed for analysis, which involves the chemical separation of the bound substance of interest from the binding reservoir and thereafter undertaking qualitative and/or quantitative measurement of the substance of interest by conventional laboratory techniques.
Another quantitative sweat collection patch is disclosed in U.S. Pat. No. 4,756,314 to Eckenhoff. This patch uses a diffusion rate-limited membrane as a means to maintain a constant flow of fluid into the patch. The patch comprises an impermeable outer boundary structure, and is therefore an occlusive patch.
However, prior art diagnostic test patches are generally only useful for determining the presence of analytes which are present in sweat in relatively high concentrations, such as halide ions. In addition, the occlusive outer layer-type devices of the prior art are susceptible to the problem of back diffusion of perspiration and/or the analytes contained therein, including changes in the skin's transport characteristics, both outward (Brebner, D. F., J. Physiol, 175: 295-302 (1964)) and inward (Feldmann, R. J., Arch. Dermat., 91: 61-666 (1965)). The maintenance of this aqueous state also fosters bacterial colonization. Thus, there remains a need in many diverse applications for an improved method and apparatus for the non-invasive determination of the presence or concentration of an analyte in a body fluid such as perspiration.